Inkjet and electrophotographic printing apparatuses have been proposed as a printing apparatus in which a print command sent from a data processing apparatus such as a personal computer is processed as a visible image on printing paper.
According to the electrophotographic method, a uniformly charged photosensitive body is irradiated with an optical image corresponding to image data of one page. For this purpose, by ON/OFF-driving and controlling a semiconductor laser in accordance with a print command, the surface of the photosensitive body is irradiated with an optical image of a laser beam to form an electrostatic latent image. In order to visualize the electrostatic latent image, it is developed with toner serving as a coloring material. A printing process is executed to transfer the developed toner image onto printing paper. The electrophotographic printing apparatus as a page printer executes a printing process in accordance with a print command which expresses image data of one page.
In contrast to the page printer, there is proposed a raster printer which directly prints image data of each line or a plurality of lines as a visible image on printing paper. The raster printer can execute a printing process if image data of necessary lines are input as a print command without inputting image data of one page.
Hence, the raster printer can execute a printing process only with a raster memory corresponding to one line or a plurality of lines without any page memory which stores print commands of a plurality of pages. The raster printer can minimize the memory capacity, at the same time, can directly print on printing paper, can be downsized, and can reduce the cost.
Such a raster printer is typified by an inkjet printing apparatus. The inkjet printing apparatus directly forms dots (pixels) of ink on printing paper and obtains a visible image by scanning in a direction perpendicular to the printing paper feed direction an inkjet head having at least orifices for discharging ink.
A function generally called duplex printing is realized by forming visible images on both the upper and lower surfaces of printing paper. Duplex printing operations are roughly classified into two types. One is automatic duplex printing of forming visible images on the two surfaces of printing paper by the printer itself without the mediacy of the user. The other is manual duplex printing of forming a visible image on one surface of printing paper, then manually setting at the delivery port again the printing paper having undergone printing on one surface, and forming a visible image on the other surface.
Many inkjet printers are not equipped with a mechanism for performing automatic duplex printing because they aim at downsizing and cost reduction. Such a printer realizes duplex printing by manual duplex printing which requires the user's manipulation.
Manual duplex printing is achieved by a method which requires the user's manipulation only once in one printing, and a method which requires the user's manipulations equal in number to printing paper sheets used.
According to the former method, after printing is done on one-side surfaces of all paper sheets used for printing, the delivered printing paper sheets having undergone printing on only the one-side surfaces are manually set at the delivery port, and printing is done on the other-side surfaces of all the printing paper sheets. That is, printing is done on the upper surfaces of all printing paper sheets, and then on their lower surfaces. This method will be called normal manual duplex printing.
To the contrary, according to the latter method, after printing is done on one surface of one printing paper sheet, the delivered printing paper sheet having undergone printing on only one surface is manually set at the delivery port, and printing is done on the other surface of the paper sheet. This procedure is repeated by the number of printing paper sheets. That is, printing is done on the upper surface of one printing paper sheet, and then on its lower surface. This method will be called manual-feed manual duplex printing.
In manual duplex printing, printing paper having undergone printing on one surface side must be manually set again at the delivery port. In order to enhance user friendliness, normal manual duplex printing is often employed in which the user's manipulation is necessary only once in one printing.
In normal manual duplex printing, it takes a long time for the user to set, at the delivery port again, printing paper sheets having undergone printing on one surface after printing is done on one-side surfaces of all paper sheets used for printing. The user instructs the printing system on a timing when printing is done on the other surface. As a method of simplifying even the user instruction and further improving convenience, there is proposed a method of automatically starting printing on printing paper supplied again in the printing system on the basis of the timer (Japanese Patent Laid-Open No. 9-216431).
However, the above-mentioned prior art suffers the following problems.
Since the inkjet printer forms a visible image by discharging ink onto printing paper, a long time is taken until ink attached onto the printing paper dries. In normal manual duplex printing employed for manual duplex printing of many inkjet printers, before ink attached to a printing paper sheet completely dries, printing is done on the next printing paper sheet, and delivered printing paper sheets are stacked over each other. The progress of drying of ink changes depending on a portion of the surface of printing paper. This results in density nonuniformity, color staining, and the like in a visible image on a printed surface.
In many cases, a purpose of manual duplex printing using plain paper is document printing, and no photographic quality is required. The above problems can be somewhat solved by decreasing the ink amount used to form a visible image in printing. However, the photographic quality is required for a visible image when manual duplex printing is done using double-sided glossy paper or the like which makes it possible to print on the two surfaces of printing paper at the photographic quality. For this reason, the ink amount used cannot be simply reduced, and problems such as density nonuniformity and color staining arise more seriously than on plain paper.
A printer which always performs manual-feed manual duplex printing can output, one by one, printing paper sheets each bearing visible images on the two surfaces, and the user can dry the paper sheets having undergone printing on the two surfaces so as to prevent them from overlapping each other. However, the user must set all printing paper sheets at the delivery port one by one, and user friendliness becomes much poorer than in normal manual duplex printing. Especially when a large number of document pages is subjected to manual-feed manual duplex printing using plain paper, manual-feed manual duplex printing is not practical because a large number of printing paper sheets must be set at the delivery port one by one.